1. Field of the Invention
The present invention relates to methods and apparatus for sealing the top of oil field casing to circulate fluid through the casing and into the well bore, and more particularly relates to simple and reliable techniques for sealing and circulating fluid through the casing utilizing equipment which may also be employed to fill the casing with fluid as it is being run into the well.
2. Description of the Background
In the oil recovery business, casing is commonly used as the outer conduit in a well bore, while tubing and drill pipe are generally positioned within the casing. When lowering casing into a well bore (also called running casing into a hole), it is not unusual to encounter difficulty in getting the casing to the desired depth in the hole, especially when the bore hole is highly deviated. Typically, casing is "unstuck" in a hole by circulating fluid, such as drilling mud, down the casing to wash sand or other debris away from the vicinity of the lowermost end of the casing.
In most instances, this circulation requires the operator to "rig-up" on top of the uppermost end of the casing string, which typically is from 10' to 40' above the drilling rig floor. Since the position of the upper end of the casing string for this rig-up operation varies with the depth of the stuck casing in the well, the height at which rig-up occurs varies and the uppermost end of the casing string is thus not readily accessible to the operator. Accordingly, an operator is frequently suspended in the air from a safety harness to perform "rig up" on the uppermost threaded end of the casing string. Once rig-up has occurred, drilling fluid is typically circulated into the top of the casing string to achieve the washing operation, at which time the casing can be further lowered simultaneously or subsequently into the well bore.
In a conventional rig up operation, a circulating sub is threaded onto the uppermost threads of the last joint of casing added to the string. Since this procedure commonly takes 30 minutes or more and, once the casing becomes stuck in the well bore, each successive joint of casing is likely to become restuck while the casing is run into the well, hours of valuable rig time and effort are required simply to attach and detach this circulating sub from the top of the casing string so that fluid can be circulated for the washing operation and the casing again lowered into the well.
Another common problem with lowering casing into a well bore is related to getting fluid into the casing as it is lowered into the well bore. While a check valve is typically provided at the lowermost end of the casing string to prevent well fluid from entering the interior of the casing as it is run into the well, fluid must be added to the lowered casing to prevent collapse of the casing during the run in operation. In most instances, a fill line from the existing mud pumps is manually placed into the exposed upper end of the casing string to fill the casing at desired intervals with fluid. This procedure typically takes several minutes per joint of casing, which amounts again to a considerable loss of rig time while running casing into a deep well.
The benefit of more quickly inputting a fluid into casing positioned in a bore hole has long been recognized in the industry, and accordingly others have devised techniques for sealing the top of the casing string without threading a tool to the casing. Various types of quick couplers have been devised for positioning at the upper end of a casing string to circulate cementing fluid through the casing. These devices are not, however, generally suitable for circulating drilling or washing fluid into casing, since it still takes a considerable amount of time to properly install the coupler on the uppermost end of the casing string. Moreover, these devices do not overcome the previously mentioned problems concerned with the safety risks and an expense associated with circulating fluid into a casing by positioning an operator at varying heights far above the rig floor. Finally, these quick couplers do not solve the problems associated with easily filling the casing with drilling mud as a casing is run into the well.
The disadvantages of the prior art are overcome by the present invention, and improved methods and apparatus are hereinafter disclosed for easily and reliably circulating fluid through casing and into a well bore and/or filling casing with fluid as it is lowered into the well bore.